Epidemiology — M. perstans is endemic in a large portion of sub-Saharan Africa, from Senegal to Uganda and south to Zimbabwe, and in Central and South America, from Panama to Argentina (figure 1 and figure 2) [1]. Since the majority of infected individuals are asymptomatic and diagnosis is typically based on morphologic examination of blood microfilariae, the epidemiology of M. perstans has not been clearly defined. Nevertheless, it has been estimated that 114 million people may be infected and as many as 581 million people in 33 countries are at risk for M. perstans infection in Africa alone [1]. Of note, molecular analyses suggest that reports of atypical blood microfilariae in residents of the Amazon regions of Brazil and Peru, which were originally thought to be M. perstans, are actually M. ozzardi [2,3]. (See 'Mansonella ozzardi' below.)

In endemic regions, the probability of infection increases with age, with prevalences reaching 100 percent in highly endemic areas. Infection of travelers is uncommon, but it does occur [4]. Nonhuman primates are occasionally infected, but they do not appear to be a major reservoir of infection.

M. perstans is transmitted by biting midges (Culicoides). The life cycle is similar to that of other filariae (figure 3). Infective larvae introduced during the bite of an infected midge mature over months into adult worms. The adult worms live in the pleural, pericardial, and peritoneal cavities as well as the mesenteric, perirenal, and retroperitoneal tissues, where they produce unsheathed microfilariae that are found in the bloodstream at all times. Microfilariae are responsible for transmission of infection because they are taken up during the blood meal of the insect vector. The life cycle is completed following maturation of microfilariae into infective third-stage larvae within the midge. The lifespan of the adult worms is unknown, although microfilaria-positive cases have been reported up to 10 years after the infected individual has left the endemic area [5].

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